Digital Inkjet Printing

16 Journal of the HKPCA / Issue No. 30 / 2008/ Q4

Technical Paper

MacDermid Electronics Solutions, Waterbury, CT USA 06702 by: John Ganjei, David Sawoska, Andrew Krol

Briefing on the Ink Jet technologyBriefing on the Ink Jet technology

Data in vector formatCAM data like

GERBER, DPF oCAD

Raster Process

Digital imageBitMap

Dpi

Abstract

Introduction

Using digital inkjet printing technology to directly

pattern various substrates for chemical engraving

processes is becoming a viable alternative to

current technologies. Leading-edge, commercially

available inkjet printers equipped with state-of-

the-art piezoelectr ic drop-on-demand (DOD)

printheads and UV exposure units allow users to

directly print and cure UV etch resists. The

elimination of the processing steps of artwork

generation, imaging, exposure, and development

used for photolithorgraphy and screen printing

technology is realized. The reduction in equipment,

materials, and cycle time equals lower production

costs. The development efforts to date with a

commercially available inkjet printed and a new

etch resist will be presented.

The driver for using a digital inkjet printer to put

down a UV etch resist for PCB innerlayer fabrication

is pretty straight forward the elimination of all

photolithography processes and equipment, i.e.

artwork generation, photoresist exposure and

development. The reduction in space requirements,

material usage, cycle time, and associated costs

makes the switch to digital inkjet printing very cost

effective. This article presents the latest results at

achieving this goal.

The photolithography process has been and

c o n t i n u e s t o b e t h e w o r k h o r s e o f P C B

manufacturing. As track dimensions and tolerances

have become smal le r and t ighter, process

improvements have been able to keep pace. But

with manufacturing costs being factored more and

more into the equation, reducing processing steps

and cycle time is a major focus of all PCB

fabr icators . As the fo l lowing process s tep

comparison demonstrates, digital inkjet printing of

a primary etch resist is very advantageous.

Digital Inkjet Printingfor Etching Circuits

By directly translating the design f ile into a print

raster f ile, the inkjet printer allows the elimination

of the artwork department in innerlayer etching as

shown in Table 1.

Photolithography Inkjet

Prepare artwork and proof

Prepare surface Prepare surface

Coat photoresist

Image photoresist Print UV resist

Develop photoresist

Etch photoresist Etch resist

Table 1

Laser direct imaging (LDI) is currently the most

advanced digital imaging method being used for

PCB manufacturing. LDI dry f ilm photoresists are

capable of sub 50-micron resolution with good edge

def inition. While this technology eliminates the

artwork generation step in the above table, it still

requires all the other photolithography process

s t e p s . M o r e i m p o r t a n t l y , t h e L D I

equipment/maintenance and the photoresists are

very expensive.

T h e c o n c e p t o f i n k j e t p r i n t i n g f o r P C B

manufacturing dates back over 20 years. Currently

inkjet printers have only been successfully used

commercially for legend or nomenclature printing

due to the lower requirements for print resolution

17www.hkpca.org

Technical Paper

and def inition. The high-resolution requirements of

PCB innerlayer imaging have traditionally outpaced

the capabilities of inkjet printhead and printer

technology. In the last year, however, the evolution

of piezoelectric drop-on-demand (DOD) printhead

technology has resulted in precise and repeatable

drops, with volumes down to 3 picoliters (10 liters).

Next, the printhead and printer have to be fully

integrated to accurately reproduce the digital

image onto the c i rcu i t board. F ina l ly, the

development of etch resist inks which are

compatible with the materials of construction of the

printhead and achieve good drop formation is

critical for f ine line resolution.

This paper presents results achieved to date with a

newly developed UV curable etch resist, MacDermid

CircuitJet 100, on a commercially available inkjet

printer. In order to print 100 and sub 100-micron

traces, the inkjet printer, printhead, ink, and

substrate surface interactions must all be optimized.

The printing platform design has to meet the

required needs of the PCB manufacturing process:

1. Rigid and flex capability

2. Image translation software

3. Substrate clamping system

4. XY positioning accuracy

5. Printhead alignment and height adjustment

6. Front-to-back alignment

7. Integrated UV curing

8. Automatic ink delivery system

9. Printhead cleaning/maintenance station

The "New Print" line of etch resist inkjet printers,

commercially available from New System S.r.L.,

Gorizia, Italy, will meet these requirements. The

New Print UV Curable InkJet G4SL (Giga) Model

Etch Resist Printer is shown in Figure 1.

Along with the positioning accuracy and image

translation software of the printer, the choice of

printhead technology is crucial to placement

accuracy and printed image quality. In the case of

PCB manufacturing, piezoelectric drop-on-demand

-12

The Printer

Figure 1

(DOD) printhead technology, which produces

precise and repeatable drop volumes with minimal

angular deviation, is the preferred technology.

State-of-the-art DOD printheads can eject drop

volumes as small as 3 picoliters (pL).

The New System printer is currently equipped with

a variable greyscale DOD inkjet head that can print

drop sizes of 6 pL to 42 pL in volume. This allows

the printing machine to match drop size with

required resolution. As a result, large features such

as power/ground can be printed at lower resolution

and larger drop size, thereby increasing overall

print speed.

Panel throughput will depend on a number of

factors, such as print speed, number of printheads,

and resolution/dpi. The near term throughput goal

is 50 - 60 sides/hour, with a future goal of 120

sides/hour. Print speed and drop placement has the

greatest effect on the printed image quality.

Increasing the number of printheads and/or the

native resolution (nozzle spacing) of the printhead

versus increasing the print speed will result in

better image quality with faster throughput.

In regards to flex manufacturing, inkjet printing

can be easily interfaced into the production process.

Another New Pr int model in the stage of

development is a reel-to-reel version enabling

direct flex circuit printing. An earlier version is

shown in Figure 2.

18 Journal of the HKPCA / Issue No. 30 / 2008/ Q4

Technical Paper

UV Curable Etch ResistThe chemical properties of the resist have to be

optimized for inkjet printing and PCB fabrication.

The ink must be compatible with the materials of

construction of the printhead and stable over time

and at operating temperature. The viscosity and

surface tension of the ink will affect nozzle plate

build-up, drop formation, and drop spreading.

Varying the jetting temperature and voltage will

influence these parameters, but only optimizing the

ink chemistry will result in good drop formation.

Figure 3 shows the results of ink formulation

optimization using a Drop Watcher III developed by

iTi, Boulder, CO. Figure 3A shows an ink with poor

drop and tail formation leading to the formation of

satellites. These satellites will lead to excess

copper and poor line formation. Figure 3B shows

good drop formation with optimized ink chemistry.

Figure 2

A B

Figure 3

In addition to the native drop size of the printhead,

the actual printed resolution also depends on the

drop dynamics of ink rheology, surface roughness

and UV response. Once optimized, drop spreading

can be minimized.

Figure 4 shows the effect of improving the

substrate/ink interaction in the CircuitJet process.

The top image shows a precleaned copper

innerlayer with line growth using typical non-

optimized inkjet printing. The bottom image shows

the results using the CircuitJet optimized process

with the identical drop volume.

Figure 4

The comparison of the lines show that the

conventional surface and printing process gives a

thin < 2 micron coating with 200 micron linewidth

while the CircuitJet process achieves a 8-10 micron

thickness coating at 60 micron linewidth.

19www.hkpca.org

Technical Paper

Figure 5 illustrates etched 150-micron width

circuits typical of large drop printers presented in

previous inkjet papers with an non-optimized

process. This type of feature def inition is not

acceptable for an etch application. Along with

optimizing the jetting properties of the ink and its

surface interaction, the formulation must also be

functional for circuit board manufacturing, i.e. fast

cure response, good adhesion, etch resistance, and

strip in conve